Chang, C.-H. Chang, C.H. Chung, T.Y. Hwang, C.-S. Kuo, C.Y. JACoW Publishing Geneva, Switzerland A Twin-Helix Undulator (THU) with a period length of 24 mm was designed to produce strong and homogeneous helical fields within a 5 mm diameter aperture. A 10-period prototype was constructed to test mechanical and magnetic performance characteristics. The prototype is composed of four helically shaped magnet arrays, which are designed to produce helical and linear fields by shifting its diagonal magnet arrays. A quadrant and half of the magnetic arrays are evaluated to verify the magnetic field performance. Helical array girders were assembled in a rectangular box-like frame with a cross-roller guideway. Mechanical considerations and field analysis of the THU are discussed in this note.